Over-voltage protection devices can be used for protection of, for example, a TMA (Tower Mounted Amplifier) 102 mounted on a mast top of a BTS/Node B 101, as is shown in FIG. 1.
One type of such over-voltage protection devices is previously known from an over-voltage protection device as the one showed in FIG. 2.
In the background art protection device 200, shown in FIG. 2, a DC input 201 receives supply voltage for an electronic device being connected to the DC output 202 of the protection device 200. The electronic device connected to the DC output 202 can, for example, be a TMA. The background art protection device 200 has further a resistance 204 connected between the conductors of the DC input 201 and the DC output 202. This resistance should have quite low resistance, for instance 1Ω.
The background art protection device shown in FIG. 2 further includes a two electrode GDT (Gas Discharge Tube) 203 connected between the conductor of the DC input 201 and ground for primary surge protection. A GDT has very high impedance under normal operating conditions, in the order of 1 TΩ in parallel with 1 pF, or less. When the voltage rises to a predetermined value, an actuation voltage, the impedance drops abruptly and a current starts to flow through the gas of the GDT. Increasing currents causes the gas to form a plasma, causing the voltage across the GDT to drop further, to around 15 V. The plasma extinguishes itself when the current through the GDT decreases.
A transorb diode 205, constituting a secondary surge protection, is connected between the conductor of the DC output 202 and ground. The transorb diode 205 is non-conducting (reverse-biased) during normal conditions and is conducting (forward-biased) when a voltage on the conductor of the DC output 202 rises over a predetermined value. The transorb diode 205 takes care of transient voltage peaks appearing on the conductor of the DC output. The secondary surge protection may also include a number of transorb diodes and resistances connected in series between the DC output 202 and ground.
The combination of a primary and a secondary surge protection is commonly used in over-voltage protection devices. The reason for this is that the different circuits used for primary and secondary surge protection, the GDT and the transorb diode, have different advantages. The GDT used for primary protection can handle very high currents, but is not very fast. The transorb diode used for secondary protection is very fast, but can not handle very high currents. By combining the high current and relatively slow GDT with the faster and lower current transorb diode, the advantages of both these circuits can be combined and results in a good total protection characteristic.
The background art protection system relies on the GDT of the primary surge protection to handle the main part of the current, by letting the current flow through the gas of the GDT, when a strike of lightning causes the voltage over the GDT to rise enough for the GDT to start conducting.
However, there is a problem in background lightning protection devices, such as the one shown in FIG. 2, regarding the function of the GDT. GDTs are manufactured to have a specified actuation voltage, for which voltage the GDT should start conducting. In practical use, however, the actual actuating voltage needed to trigger the GDT depends on the shape of the over-voltage pulse appearing at the DC input 201 of the device. It is not at all sure that the GDT starts conducting when it is supposed to. There is thus a risk that the GDT starts to conduct very late, when the voltage has already reached a much higher value than the specified actuation voltage value.
This late actuation can have the effect that a much bigger part of the lightning energy than the transorb diode has been designed for reaches the secondary surge protection. The secondary surge protection may not be able to handle this energy and the circuits meant to be protected might therefore be in danger.
Background art solutions have been presented that aim to solve this problem. Such background art solutions are shown by GB 2 166 307 A and FR 2 544 923 A1. These solutions utilize a triggering function of a voltage arrester in order to trigger the voltage arrester to start conducting.
The basic idea of these background art solutions is to let a transformer create a voltage, by transforming a voltage present on the input of the protection device, and to use this created voltage for triggering the voltage arrester into a conducting state. These solutions make great demands on the characteristics of the transformers. They have to have a low permeability (μ) core and a high number of windings. There is further a risk that the iron core of transformer might be saturated.
Background art document GB 1 467 318 shows further a semiconductor converter device, in which a surge arrester is triggered by a pulse created by discharging a capacitor. When creating the triggering pulse in a controlled manner, as is shown in this background art solution, the requirements of the transformer can be lowered.
However, the creation of the triggering pulse according to this background art solution is very complex. A large number of circuitry is here needed for creating the triggering pulse, including a number of DC voltage sources, that might have to be substituted when they have to be charged or when they are out of order. Substituting such DC voltage sources might take some time to do and during this time the surge protection does not work, resulting in a protection hazard, if over-voltage would occur during this time.
Also, over-voltage protection devices may be located in a mast top in the wilderness, and to perform service at such location can be both time consuming and expensive. As is well known, complex devices are more likely to need service than simple and robust devices, and the complex background art solutions are therefore prone to errors that need service.
Thus, in background art over-voltage protection devices, there exist problems relating to strict demands on transformers used and also problems relating to operation and complexity of the devices. These problems further lead to expensive over-voltage protection.